U.S. Pat. No. 4,678,291 discloses an optical apparatus for microscopes. The apparatus makes possible a rapid changeover between two different observation methods of the microscope. The apparatus encompasses a dovetail guide on which the Bertrand lens is guided. The Bertrand lens can be shifted in the beam path by way of a threaded spindle. The threaded spindle makes possible accurate positioning of the Bertrand lens. It must be noted here, however, that the Bertrand lens is continuously positioned in the beam path, and in the case of the apparatus described here no provision is made for pivoting the Bertrand lens in and out.
GB Patent 590,665 describes an improvement for microscopes, in which the Bertrand lens is provided in the beam path of the microscope between the objective and the eyepieces. The Bertrand lens is secured at opposite ends in a ring-like holding element, and the holding element can be pivoted into the beam path of the microscope. Also associated with the Bertrand lens is a diaphragm that, in an embodiment of the invention, can be adjusted from outside. Adjustments of the diaphragm associated with the Bertrand lens can be performed irrespective of the position of the annular holder, i.e. an adjustment is possible even when the Bertrand lens is not in the beam path.